1. Field of the Invention
The present invention relates to a method and apparatus for detecting the level of a liquid.
2. Related Background Art
In the fabrication of semiconductor devices, various wafer processing systems are utilized to remove surface contamination such as small particles, organic contaminant materials, metallic impurities or the like from a semiconductor wafer (referred hereinafter to as xe2x80x9cwaferxe2x80x9d) using a processing liquid such as chemicals, pure water and the like. A wet-type processing system, among others, has been widely used wherein the wafers are submerged in a processing tank or bath containing the processing liquid for cleaning.
In such a processing system, it is necessary to fill the processing tank with a selected amount of the processing liquid enough to perform an effective cleaning operation. To this end, a liquid level detector apparatus is used in conjunction with the wafer processing tank for detecting the level of the liquid being supplied in the tank and terminating the supply of the liquid as the liquid level reaches a predetermined position or elevation in the tank.
The liquid level detector apparatus comprises a hollow sensor tube having an open distal bottom end positioned at a predetermined height I the tank and adapted to be immersed in a processing liquid whose level are adapted to be detected, gas supply means for supplying gas having a pressure substantially greater than a pressure prevailing over the liquid upper surface to the sensor tube sufficiently to fill the interior thereof entirely to a depth of the open distal bottom end, a pressure sensor associated with the sensor tube for generating an output signal representative of the gas pressure within the sensor tube and an electrical circuitry for determining the liquid level based on the magnitude of the output signal from the pressure sensor. Thus, the liquid level detecting apparatus is configured for detecting the liquid level based on the gas pressure measured at the predetermined position (i.e., at the predetermined depth) in the processing tank.
More particularly, with the open distal bottom end of the sensor tube immersed in the processing liquid in the tank, the gas supply means supplies the gas such as compressed air or an inert gas having a pressure substantially greater than a pressure (e.g., atmospheric pressure) on the liquid surface into the hollow interior of the sensor tube to a depth level where the open distal bottom end thereof is located, so that the gas pressure within the sensor tube is equal to the liquid pressure being applied to the open distal bottom end of the sensor tube. It is to be understood that the liquid level detector apparatus of the type as outlined above is capable of detecting the liquid level relying on the fact that the gas pressure within the sensor tube is proportional to the distance (depth) measured from the liquid surface to the distal bottom end of the sensor tube. On the other hand, in view of the fact that the distal bottom end of the sensor tube is located at a predetermined height or a known depth in the processing tank, the pressure of the gas being introduced into the hollow sensor tube is equal to the pressure (e.g., atmospheric pressure) on the liquid surface until the level of the processing liquid being supplied comes into contact with the distal bottom end of the sensor tube. During that time period, the pressure sensor will generate an output signal having a constant level or magnitude. Such a pressure-detected output signal generated by the pressure sensor under this condition can be set as a reference signal.
The liquid level will rises as the processing liquid is fed into the processing tank. As can be appreciated by those skilled in the art, the pressure sensor continues to provide the reference pressure signal until the liquid level reaches the level (the predetermined position) where the open distal bottom end of the sensor tube is positioned. When the output signal from the pressure sensor exceeds the reference signal level, this event indicates that the liquid level reaches the predetermined position in the processing tank. The magnitude of the pressure-detected output signal from the pressure sensor rises linearly as the level of the liquid being supplied rises in the tank. The electrical circuitry operates to detect the depth of the distal bottom end and thus the liquid level based on the level of the pressure-detected output signal from the sensor. The pressure-detected output signal of the predetermined level generated by the pressure sensor indicates that the liquid level reaches the predetermined level in the processing tank. This allows an immediate termination of the supply of the processing liquid to the tank.
The pressure sensor may comprise any conventional pressure transducer of the type having a diaphragm displaceable depending on change in the gas pressure and a strain gauge adapted to convert the displacement of the diaphragm into an electrical signal. By way of example, a highly sensitive piezoresistive semiconductor pressure sensor may be used which comprises a diaphragm formed in an N-type silicon substrate and a P-type strain gauge fabricated on a portion of the diaphragm by diffusing a boron layer therein.
The pressure sensor may experience drift under the influence of various environmental conditions (e.g., temperature and humidity) and/or deterioration with age and produce a reference signal drift. For example, in case where the pressure sensor erratically generates a reference signal having a level higher than the normal signal level before the distal bottom end of the sensor becomes immersed in the processing liquid in the cleaning tank, the pressure sensor will generate a signal which is up-shifted relative to a pressure-detected signal level indicative of an actual gas pressure prevailing within the sensor tube at the time when the distal bottom end of the sensor tube becomes immersed in the processing liquid. Under such condition, the pressure sensor will provide prematurely an output signal of the predetermined level to terminate the supply of the processing liquid to the tank before the liquid level reaches the predetermined level in the processing tank, which may lead to reduced cleaning capability due to shortage of the processing fluid. In contrast, when the pressure sensor erroneously generates a reference signal having a level lower than the normal signal at the time the distal bottom end of the sensor tube becomes immersed in the processing liquid, the pressure sensor will produce an output signal having the predetermined level after the liquid level actually reaches the predetermined level or position. As a result, the actual liquid level may ascend past the predetermined level in the tank, which will result in waste of the processing fluid due to excessive supply thereof to the processing tank.
It is therefore an object of the present invention to provide an apparatus and method for precisely detecting the level of a liquid in spite of any sensor output signal drift.
With the object in view, a first embodiment of the present invention relates to an apparatus for detecting the level of a processing liquid in a processing tank, comprising detector means (e.g., detecting sensor) for generating an output signal related to the level of the liquid being supplied to the tank; a determination circuit for determining when the liquid level reaches a first position established in the tank on the basis of the output signal from the detector means; a liquid level detecting circuit for correcting a relationship between a level of the output signal from the detector means and the liquid level based on a signal generated by said detector means from the time the processing liquid is initially supplied to the tank until immediately after the liquid level reaches the first position to thereby detect the actual liquid level in the tank.
In accordance with the first embodiment of the liquid level detecting apparatus according to the invention, the processing liquid is supplied to the processing tank and the level of the liquid rises in the tank accordingly. The detector means generates an output signal related to the level of the liquid being supplied to the tank. More particularly, the detector means generates an output signal having a constant level or magnitude until the liquid level reaches a first position or level established in the tank. The detector means, which properly operates, generates an output signal from the time the processing liquid is initially supplied until immediately after the liquid level reaches the first position, is set as a reference signal. As the liquid level reaches the first position level, the magnitude of the output signal from the detector means will exceed the reference signal level. From this event, the determining circuit can determine whether the liquid level reaches the first position. At that time, the amount of the processing liquid within the tank can be determined because the depth of the predetermined position is known. As the liquid level continues to rise past the first position, the level of the output signal from the pressure sensor rises accordingly. It is noted that the signal level indicates the magnitude of the pressure-detected signal.
When the pressure sensor may experience drift resulting from the environmental conditions (e.g., temperature and humidity) and/or deterioration with age, the detector means will generate a pressure signal whose level is either up-shifted or down-shifted with respect to a signal level representative of the actual liquid level in the tank. However, in the present invention, the liquid level detecting circuit functions to correct the output signal from the detector means on the basis of a reference signal provided by the detector mans from the time the processing liquid is initially supplied to the tank until immediately after the liquid level reaches the first position to provide a signal correctly representing the actual liquid level, which signal is substantially similar to the signal obtained when no drift occurs in the detector means during detection of the liquid level within the tank. More specifically, the output signal form the detector means can be corrected by compensating for the difference in signal level between the reference signal (known value) and the pressure-detected output signal from the detector means till immediately after the liquid level reaches the first position from the bottom level of the tank. Thus, the liquid level of the processing liquid in the tank can be correctly determined even if the detector means can experience the drift.
In accordance with a second embodiment of the invention, preferably, the liquid level detecting apparatus further comprises a hollow sensor tube having an open distal bottom end positioned at the first position and adapted to be immersed in the processing liquid and gas supply means for supplying gas having a pressure substantially greater than a pressure prevailing over the liquid upper surface to the sensor tube sufficiently to fill the interior thereof entirely to a depth of the open distal bottom end, the detector means providing the output signal representative of the gas pressure within the sensor tube, the determination circuit comprising a high-pass filter for extracting ripple in the output signal to generate an AC signal and a signal processor for rectifying the AC signal from the high-pass filter.
With this arrangement, the gas supply means supplies the gas such as compressed air or an inert gas having a pressure substantially greater than a pressure (e.g., atmospheric pressure) prevailing over the liquid surface into the hollow interior of the sensor tube to a depth level where the open distal bottom end thereof is located, so that the gas pressure within the sensor tube is equal to the pressure prevailing over the liquid surface until the liquid level reaches the first position. As discussed above, the detector means outputs a signal having a constant (flat) level to the high-pass filter of the determination circuit. The high-pass filter will generate a zero level signal because of the constant level signal from the detector means, which signal is coupled to the signal processor. Then, the signal processor will provide a xe2x80x9cLow levelxe2x80x9d signal.
The gas supply means continues to supply the gas to the sensor tube in such a manner as to fill the hollow interior of the sensor tube entirely, in other words to prevent the processing liquid from entering the open distal bottom end of the sensor tube. As a result, bubbles emerge from the open distal tube end intermittently or periodically. Such bubbling can cause fluctuation in the gas pressure in the sensor tube and as a result the detector means may generate an output signal containing an alternating component or ripple, which signal is input to the high-pass filter of the determination circuit. The high-pass filter extracts only the alternating component in the output signal to output an AC signal to the signal processor. The signal processor rectifies the AC signal to generate a xe2x80x9cHi levelxe2x80x9d signal, which indicates that the liquid level reaches the first position. Thus, the extraction of the alternating component in the signal allows the determining circuit to reliably determine whether the liquid level reaches the first position, which can provide a correct liquid level detection in spite of detector output signal drift.
As can be seen, the gas pressure prevailing within the sensor tube is equal to the liquid pressure being applied to the open distal bottom end of the sensor tube and also is proportional to the distance (depth) measured from the liquid surface to the distal bottom end of the sensor tube. The detector means can detect the gas pressure within the sensor tube to provide an output signal on the basis of which the depth of the distal bottom end of the tube and thus liquid level in the tank can be determined. Alternatively, or in a third embodiment, the detector means is operable to detect when the liquid level reaches a second position higher than the first position in the processing tank. With this arrangement, it is possible to supply a predetermined amount of the processing liquid to the tank, thereby avoiding reduction in processing capability and also waste of the processing liquid. No limit sensor on the top of the processing tank for establishing the upper limit of the liquid level in the tank, is required.
A fourth embodiment of the present invention provides an apparatus for detecting the level of a processing liquid in a processing tank, comprising detector means for generating an output signal related to the level of the liquid being supplied to the tank; a liquid level detecting circuit for comparing a signal level of a output signal generated by the detector means before the liquid level reaches the predetermined position to a predetermined reference signal level to generate a correction signal and for correcting the output signal generated by the detector means on the basis of the correction signal to detect the liquid level in the tank.
In accordance with the fourth embodiment, the processing liquid is supplied to the processing tank and the level of the liquid rises in the tank accordingly. The detector means generates an output signal related to the liquid level and outputs the signal to the liquid level detecting circuit. After the liquid level reaches the predetermined position, the detector means will generate an output signal corresponding to the liquid level rising past the predetermined position. In the liquid level detecting circuit, the signal level of the reference signal is set to be equal to that of the signal output from the properly operating detector means at the time when the liquid level reaches the predetermined position.
When the pressure sensor may experience drift resulting from the environmental conditions (e.g., temperature and humidity) and/or deterioration with age, the detector means will generate a pressure signal whose level is either up-shifted or down-shifted with respect to a signal level representative of the actual liquid level in the tank. According to the teachings of the present invention, the liquid level detecting circuit is operable to calculate the difference the level of the output signal generated by the detector means prior to the detection of when the liquid level reaches the predetermined position and the level of the reference signal to provide a correction signal representative of the difference. Thereafter, the correction signal is subtracted from (or added to) the output signal from the detector means, thereby compensating for the difference. The corrected signal level always corresponds to the actual liquid level in the processing tank. Thus, the liquid level detecting system is arranged in such a manner as to precisely determine the liquid level of the processing liquid in the tank even if the detector means could experience the drift.
A fifth embodiment of the present invention provides an apparatus for detecting the level of a processing liquid in a processing tank, comprising detector means for generating an output signal related to the level of the liquid being supplied to the tank; a determining when the liquid level reaches a predetermined position established in the processing tank to generate an output signal; a determination circuit for determining when the liquid level reaches a predetermined position on the basis of the output signal from said detector means; a liquid level detecting circuit for comparing a signal level of a output signal generated by the detector means before the liquid level reaches the predetermined position to a predetermined reference signal level to generate a correction signal and for correcting the output signal generated by the detector means based on the correction signal to detect the liquid level in the tank.
In accordance with the fifth embodiment, the processing liquid is supplied to the processing tank and the level of the liquid rises in the tank accordingly. The detector means generates an output signal related to the liquid level and provides the output signal to the liquid level detecting circuit. After the liquid level reaches the predetermined position, the detector means will generate an output signal corresponding to the liquid level rising past the predetermined position. In the liquid level detecting circuit, the signal level of the reference signal is set to be equal to that of the signal output from the properly operating detector means at the time when the liquid level reaches the predetermined position. The liquid level detecting circuit is operable to calculate the difference the level of the output signal generated by the detector means prior to the detection of when the liquid level reaches the predetermined position and the level of the reference signal to provide a correction signal representative of the difference. Thereafter, the correction signal is subtracted from (or added to) the output signal from the detector means, thereby compensating for the difference. The corrected signal level always corresponds to the actual liquid level in the processing tank. Thus, as in the fourth embodiment of the invention as described above, the liquid level detecting system is arranged in such a manner as to precisely determine the liquid level of the processing liquid in the tank even if the detector means could experience the drift.
A sixth embodiment of the present invention provides an apparatus for detecting the level of a processing liquid comprising a hollow sensor tube having an open distal bottom end positioned at a predetermined position established in the tank and adapted to be immersed in a processing liquid whose level are adapted to be detected; gas supply means for supplying gas having a pressure substantially greater than a pressure prevailing over the liquid upper surface to the sensor tube sufficiently to fill the interior thereof entirely to a depth of the open distal bottom end; detector means associated with said sensor tube for generating an output signal representative of the gas pressure within the sensor tube; a determination circuit coupled to an output terminal of the detector means for determining whether the surface of the liquid being supplied comes into contact with the open distal bottom end of the sensor tube; and a liquid level detecting circuit coupled to the output terminal of the detector means for detecting the liquid level; the determination circuit comprising a high-pass filter for extracting only an alternating component in the output signal to generate an AC signal and a signal processor for rectifying the AC signal from the high-pass filter; and the liquid level detecting circuit a low-pass filter for removing the alternating component in the output signal to generate an output signal, buffer means for storing and outputting the output signal from the low-pass filter from the time the processing liquid is initially supplied until immediately after the liquid surface comes into contact with the open distal bottom end of the sensor tube and calculating means having two input terminals, one of the input terminal being coupled to an output of the low-pass filter and the other input being coupled to an output terminal of the buffer means for calculating any difference between the output signal from the low-pass filter and the output signal from the buffer means.
A seventh embodiment of the present invention provides an apparatus for detecting the level of a processing liquid in a processing tank, comprising a first detector means disposed in the processing tank for detecting change in pressure in a first hollow sensor tube to which gas is supplied to generate a first output signal representative of the change, the sensor tube having an open distal bottom end; a first determination circuit for determining when the liquid level reaches a first predetermined position established in the tank on the basis of said output signal from said first detector means; a liquid level detecting circuit for correcting a relationship between a level of said output signal from said first detector means and the liquid level on the basis of a signal generated by said first detector means at the time the liquid level reaches the first predetermined position to thereby detect the liquid level in the tank; a second detector mans disposed in the processing tank for detecting change in pressure in a second hollow sensor tube to which gas is supplied to generate a second output signal representative of said change, said second sensor tube having an open distal bottom end; a second determination circuit for determining when the liquid level reaches a second predetermined position defined in the tank on the basis of said output signal from said second detector means.
An eighth embodiment of the present invention relates to a method for detecting the level of a processing liquid in a processing tank, comprising the steps of: supplying the processing liquid to the processing tank; determining whether the liquid level reaches a predetermined position established within the processing tank to generate an output signal by means of detector means disposed in the processing tank and generating an output signal in response to said determination; detecting when liquid level reaches said predetermined position; and correcting a relationship between a level of said output signal from said detector means and the liquid level on the basis of an output signal generated by said detector means at the time the liquid level reaches the predetermined position to thereby detect the liquid level in the tank.
A ninth embodiment of the present invention provides a method for detecting the level of a processing liquid in a processing tank, comprising the steps of: supplying the processing liquid to the processing tank; determining whether the liquid level reaches a predetermined position established within the processing tank to generate an output signal by means of detector means disposed in the processing tank and generating an output signal in response to said determination; detecting when liquid level reaches said predetermined position; and correcting a relationship between a level of said output signal from said detector means and the liquid level on the basis of a signal generated by said detector means at the time the processing liquid is initially supplied to thereby detect the liquid level in the tank.
The liquid level detecting methods as constructed above correct the relationship between a level of the output signal from the detector means and the liquid level based on the signal level obtained at the time when the liquid level reaches the predetermined position in the case of the first-mentioned method and the signal level obtained at the time when the processing liquid is supplied to the processing tank in the case of the second-mentioned method. In either case, the liquid level can be correctly detected in spite of the drift. The eighth and ninth embodiments can be successfully carried out by the first and fourth to seventh embodiments.
A tenth embodiment of the invention relates to a method for detecting the level of a processing liquid in a processing tank, comprising the steps of supplying the processing liquid to the processing tank; detecting the liquid level by means of detector means disposed in the processing tank for detecting change in pressure in a hollow sensor tube to which gas is supplied; and generating an output signal representative of said change in pressure in said sensor tube.
According to this method, the gas is supplied under pressure to the processing tank. The gas pressure within the sensor tube is constant and equal to the pressure prevailing over the liquid surface until the liquid level comes into contact with the distal bottom end of the sensor tube. When the liquid level reaches the open distal bottom end of the sensor tube, bubbles begin to emerge intermittently or periodically from the open tube end, which can cause variations in the gas pressure within the sensor tube. The gas pressure within the sensor tube is equal to the liquid pressure acting across the open tube end and is proportional to the distance (depth) measured from the liquid surface to the distal bottom end of the sensor tube. Thus, it is possible to detect the liquid level in the tank by providing an output signal in response to the pressure variations. The tenth embodiment can be preferably carried out by the second and third embodiments of the liquid level detecting apparatus according to the present invention.